Heretofore, a PS plate comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin has been widely used as a lithographic printing plate precursor. As the usual plate-making method thereof, the plate printing plate precursor is subjected to mask exposure (flood exposure) through a lith film and then the non-image area is dissolved away to obtain a desired printing plate.
In recent years, digitization technology of electronically processing, storing and outputting an image information using a computer has been popularized. To cope with the digitization technology, various new methods for outputting an image have been proposed and are actually used. This tendency has yielded the demand for a computer-to-plate (CTP) technique where a printing plate can be directly produced by scanning a highly directional light such as laser light according to a digitized image information without using a lith film. As a result, it has become an important technical concern to obtain a printing plate precursor adapted for the CTP technique.
As a lithographic printing plate precursor which permits scan exposure, a constitution has been proposed wherein a photopolymerizable composition having an extremely excellent light-sensitive speed is used as the oleophilic light-sensitive resin layer formed on a hydrophilic support and an oxygen-impermeable protective layer is further provided thereon, and is available on the market. The printing plate precursor of the above-described constitution provides desirable printing plate properties and printing performance that it makes simple and easy the development processing and provides a printing plate excellent in resolving power, ink-receptive properties and printing durability.
On the other hand, recent years have seen a remarkable progress of laser technology. For example, a semiconductor laser which uses an InGaN-based material and can continuously oscillate in the range of from 360 nm to 450 nm, has come to be put into practice. A CPT system using such short-wavelength light source would make it possible to use a light-sensitive material having a light-sensitive region in the shorter wavelength region which permits to handle under a brighter safe light. Further, semiconductor lasers can be produced inexpensively due to the structure thereof, thus being extremely preferred as a light source for the CPT system.
It has been strongly desired in this industrial field to obtain a lithographic printing plate precursor adapted for the CTP system using the semiconductor laser emitting a light of comparatively short wavelength of from 350 nm to 450 nm.
In recent years, printing plate precursors containing a triazine-based initiation system (JP-A-14-116540) or a titanocene-based initiation system (JP-A-13-42524) in the photopolymerizable layer have been disclosed for the CTP system using the short-wavelength light source. The printing plate precursor having the triazine-based initiation system is highly sensitive and can be handled under a yellow light, but involves the problem that it has an insufficient storage stability (accelerated aging for a long time: for example, at 60° C. for 3 days).
Also, for the CTP system using an infrared laser of 800 nm in wavelength as a laser light source, lithographic printing plate precursors capable of being handled in a bright room (under a yellow lamp or a white lamp) is disclosed in, for example, patent document 1 (JP-A-2002-139843).
However, in the CTP system using the infrared laser as a light source, abrasion of the printing plate surface occurs due to the strong beam exposure, and residue formed by the abrasion has been a problem. Also, since a light-sensitive material for an infrared laser is much less sensitive in comparison with a light-sensitive material for a visible light laser, it involves a problem of poor productivity.
On the other hand, as light-sensitive lithographic printing plate materials for obtaining both an enough sensitivity and storage stability, there have been known those which use a specific sensitizing dye as disclosed in patent document 2 (JP-A-2002-351065), those which use a specific intermediate layer as disclosed in patent document 3 (JP-A-2003-43692), and those which use a specific cross-linking agent as disclosed in patent document 4 (JP-A-2003-64130).
[Patent document 1] JP-A-2002-139843
[Patent document 2] JP-A-2002-351065
[Patent document 3] JP-A-2003-43692
[Patent document 4] JP-A-2003-64130